Axial hydraulic piston pumps are used in a variety of applications, whether as a stand alone pump or as part of, for example, a hydraulic transmission. The components of an axial hydraulic piston pump typically include a cylinder block, a group of pistons vertically movable within openings in the cylinder block, a swash block the underside of which the pistons bear against, and a pair of arch bushings seated within recesses having axially spaced walls on the upper surface of the swash block. The cylinder block rotates the group of pistons about a vertical axis. A control arm rotates the swash block about an axis of rotation. During rotation, the swash block slides relative to the bushings, causing the bottom of the swash block to tilt upward on one side of the rotation axis and downward on the opposite side of the rotation axis. As the cylinder block rotates, each piston moves vertically downward on the downward tilted side and vertically upward on the upward tilted side in sliding relation in the openings into and out of the cylinder block, which, in turn, causes fluid to be transferred. The dual swash block bushings are formed in a matching arch to guide the swash block about its axis of rotation in a single plane when actuated by for example the control arm input. The matching arch bushings provide a vertical force to the swash block to counter the forces produced by the piston rotating group.
Some existing swash block assemblies have various shortcomings, drawbacks, and disadvantages relative to certain applications. For example, the swash block bushings formed in matching arches along the axis of rotation do not supply any guidance for the swash block in the lateral or rotational (yaw) directions. Typically, the movement of the swash block in the lateral and yaw directions is limited by mechanical surfaces or edges that are not coated to reduce friction, for example the walls of the recesses in the swash block and/or walls of the housing in which the swash block is mounted. In turn, the contact with these surfaces under high force creates significant drag which translates to the amount of force needed to actuate the swash block through the control input by the operator. FIG. 22 shows contact stresses on the swash block in the form of different shades. The highest contact stresses, which are identified by the reference numeral 700 in FIG. 22, are at the walls where the sides of the arch bushings contact due to lateral movement of the swash block along the axis of rotation and/or yaw movement about the vertical axis.
The lack of guidance in the yaw direction also allows the swash block to move within that direction due to clearance between the edges of the bushings and the afore mentioned mechanical surfaces or edges such as walls of the swash block and/or housing. This movement is introduced by the actuation of the swash block by the control arm and is most significant when the actuation of the control arm is changed from one direction to the other. This freedom of movement of the swash block in the yaw direction allows for movement of the control arm input without a corresponding change in displacement of the pump until the contact with the mechanical surface or edge, for example walls, is made. This causes what is known as dead band, an undesirable effect in which there is an inaction or lag in action between the input of the control function and the corresponding change in what is being controlled.
Accordingly, there remains a need for further contributions in this area of technology.